Update: Co-publishers Eidos and Warner Bros. have just announced that The Joker is "an exclusive playable character" for the PlayStation 3 edition. Playable in Challenge Maps, The Joker will arrive as free downloadable content "upon release."

Original: The PS3 version of Rocksteady's Batman: Arkham Asylym will let you "play as The Joker in the exclusive Joker challenge maps," going by a PlayStation Home ad.
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A representative for the title was reportedly surprised that the ads were already up when contacted by Kotaku, which originally broke the story. However, he could offer no further details on the maps or possible exclusives for the PC and Xbox 360 editions.

All three versions of the action-detective game are slated to arrive in June.